This invention relates to neutralizing transformers useful in electric power transmission systems which depends upon communication facilities to effect proper operation and control. These transformers are used to neutralize the effect of ground potential changes and longitudinally induced voltages due to the proximity to power lines on communication lines which enter the power sub-stations.
Up to the present time, multi-wound or multi-circuit neutralizing transformers have utilized telephone type cable for the secondary or signal windings. Essentially, the cable consists of a given number of balanced twisted pairs formed or cabled into a standard telephone type cable configuration. An example of such neutralizing transformers utilizing twisted pairs is shown in the U.S. Pat. to GORDON Y. R. ALLEN, No. 3,453,494, entitled NEUTRALIZING TRANSFORMERS, granted July 1, 1969. In this patent, the twisted pairs were wound around a core and connected in such a manner that the shield of the twisted pairs acted as a primary in the transformer and, as disclosed in this patent, this primary winding was connected to ground.
Another form of neutralizing transformer has been to utilize a primary winding wound in bifilar configuration with the secondary or signal winding.
Still another alternative is the use of an external shield around the coil as the primary winding.
In any of these configurations a mutual capacitance between conductors and the capacitance of conductor to ground has been very high. Furthermore, the winding space factor caused by the twisted pairs has been very poor and led to an unacceptable size. In order, however, to attempt to reduce the winding space, it has been necessary to reduce the waste space caused by the use of the twisted pairs. This in the past has normally meant a reduction in the dimensions of the dielectric and an increase in the mutual capacitance. In other words, there was a tradeoff of alternatives -- space or capacitance, and these alternatives have heretofore been unacceptable.
Accordingly, it is a first object of this invention to provide a neutralizing transformer which overcomes all the deficiencies of the prior art transformers and this is accomplished as will be understood from the following.